(a) Field of the Invention
The present invention relates to an electronic device, and particularly to an electronic device comprising an organic compound having p-type semiconductor characteristics and performing functions of injecting or transporting holes.
(b) Description of the Related Art
Organic compounds having p-type semiconductor characteristics, which has conductance in-between insulating material and conducting material, has been widely applied to OPC drums (organic photo conductor drum) and used in a copying press or a laser printer, etc. Specifically, aryl amine-type material forms a thin film on the drum of a laser printer as a blend with polymers such as polycarbonate to constitute a hole-transporting layer. For such an application, organic compounds having p-type semiconductor characteristics must have appropriate hole-mobility, and electrical, thermal and morphological stability for hole.
Devices using the p-type organic semiconductors are largely classified into the following two groups.
The first group has an application in an OPC drum or a solar cell or a photovoltaic cell. Specifically, an outer light-source is introduced into devices to generate exiton, and the exiton is separated into electrons and holes. At this time, the p-type organic semiconductor performs the function of transporting the separated holes in devices.
The second group applies voltage to two or more electrodes to directly inject a carrier into an organic semiconductor that constitute an interface with the electrode. As examples, devices such as an organic EL (electroluminescence) that injects electrons and holes respectively and simultaneously from two electrodes and emits light, devices such as transistor that transports a carrier from a source to a drain by voltage applied to a gate to perform a function of switching, etc. can be mentioned. At this time, it is important to form a stabilized interface between the p-type organic semiconductor substances and the electrodes. Generally, since an electrode consists of metallic substances or metal oxides, the performances of devices can be significantly decreased due to an electric field applied to devices or heat applied from the outside or generated from the inside unless the interface between inorganic substances and organic substances is stabilized.
The above-mentioned arylamine-type compounds have been widely used as semiconductor substances comprising organic substances showing p-type characteristics, and oligothiophene or polythiophene are also known to show high carrier mobility when experimentally used in the manufacture of thin film transistor.
Such characteristics of the compounds allow a variety of applications thereof. Specifically, arylamine type p-type organic semiconductor used in an OPC drum is also applied to an organic light-emitting diode, and oligothiophene used in a thin film transistor is also used as a hole-injecting or hole-transporting material of an organic light-emitting diode. In addition, an organic solar cell, which uses a hole-transporting material of solid state, also employs arylamine type derivatives that are used as a hole-transporting material of an organic light-emitting diode (Adv. Mater. 12, 447, 2000). Such compatibility suggests that any organic substances having p-type semiconductor characteristics can be widely used in the manufacture of devices considering their energy levels, hole-transporting capabilities, environments, etc.
The p-type organic semiconductor used in an organic light-emitting diode performs the function of facilitating hole-injection from an anode and simultaneously transporting the injected hole to a light-emitting layer. At this time, the layer may be separated into two layers of hole-injecting layer and hole-transporting layer. In order to secure the stability of devices, substances should be selected from those capable of constituting stable interface with an anode comprising metal or metal oxide. In addition, in order to facilitate hole injection thereby enabling low voltage operation of devices, the substances should have appropriate oxidation potential and high capability of transporting the injected holes.
In order to satisfy these requirements, U.S. Pat. No. 4,356,429 discloses copper phthalocyanine and U.S. Pat. No. 5,540,999 discloses oligothiophene. In addition, quinacridone type materials disclosed in U.S. Pat. No. 5,616,427 are also known to contribute to the stability of devices.
Recently, the stability of polymeric light-emitting diode has been largely improved by separately introducing a hole-transporting layer between an anode comprising metal oxide and a light-emitting material. Particularly in thermally stable polymeric light-emitting diode, substance that is inserted between an anode and a light-emitting polymer to stabilize an interface despite high glass transition temperature and that facilitates the injection of holes can largely improve performances, particularly life and operating voltage of devices (J. Appl. Phys. 84, 6859, 1998).
The p-type organic semiconductor, when used as hole-injecting material, exhibits an effect of preventing device short of light-emitting diode that may be generated during a manufacturing process of devices, as well as an effect of prolonging life of devices. Generally, organic light-emitting diode having organic substance thickness of 100 to 200 nm may become device short due to pinholes that can be generated during organic substance deposition or at the unstable interface between anode and organic substances. At this time, the injection of stable hole-injecting layer can decrease the possibility of such devices short, and particularly the injection of thick hole-injecting layer can largely decrease such possibility.
However, in the common case where the hole-injecting material has semiconductor characteristics instead of conducting characteristics, adverse effects of increasing operating voltage may be shown according to the thickness of hole-injecting material.